Remove feature flag: member_destroy_async_auth_refresh

What does this MR do and why?

Removes feature flag: member_destroy_async_auth_refresh

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #342120 (closed)